The Immune Function Studies Core is designed to support observational studies of post-transplantation immune reconstitution. The Core will perform multi-parameter flow cytometry analyses of T-cell and B-cell differentiation, sjTREC quantitation on PBMC or flow sorted T-cell subsets, and measurements of serum levels of Blys-April TNF family immunoregulatory factors. Core personnel will also be available to aid project investigators in the analysis and interpretation of multiparameter flow cytometry data. The data from these analyses will be used to define the course and extent of post-transplantation immune reconstitution, and will be correlated with the results from clinical evaluations of immune responses, patient morbidity, and graft outcome.